Controlled emulsification using microporous membranes

Abstract

Emulsions are a vital part of many products in everyday use, such as foods, cosmetics, and even construction materials. Membrane emulsification is a technique which has been used to produce emulsions in a manner contrary to the traditional methods where droplets are broken and re-broken to make smaller and smaller droplets, and instead each droplet is individually formed at a pore on the surface of the membrane.

This research compared two of the most favoured membrane emulsification techniques; cross-flow and rotating membrane emulsification.

Two systems were built for producing emulsions using tubular microporous membranes, made from shirasu porous glass, polymer, ceramic and stainless steel. One device employed a cross-flow system providing shear to detach the nascent droplets from the membrane pores whilst the other system employed a rotated membrane to produce both shear and potentially centripetal force at the membrane surface. Both systems were used to create emulsions, and the effects of various settings of the systems were investigated.

A direct comparison between cross flow membrane emulsification and rotating membrane emulsification were achieved for the first time, as the same membranes were available for both systems. The modular interchangeable nature of the membranes in the systems also allowed direct comparison between the various different membrane types tested.

The distinct differences in the structure and materials of the membranes tested was compared, and its effects elucidated, as the different membrane types each showed different advantages and disadvantages when producing droplets.

It was shown that the membrane pore size is a major factor on the size of the droplets produced, and the membrane pore size distribution span affects the size distribution span of the droplets. Increasing the emulsifier concentration decreases droplet size, as does increasing the shear force applied to the forming droplets, either by increasing the cross-flow velocity or the rotation rate. Increasing the pressure applied to force the dispersed phase through the membrane increases flux rate, but also increases droplet size slightly. The relative viscosity of the two phases being emulsified has an effect on the droplet size; increasing the continuous phase viscosity decreases droplet size, and increasing dispersed phase viscosity increases droplet size. The systems performed equally well making water in oil, as oil in water emulsions.

Although the rotating membrane system produces lower shear rates than the cross-flow system, similar droplet diameters were produced, implying that detachment is enhanced by the rotation, showing a clear advantage to rotating membrane emulsification.

The systems were also used to produce various more complex particles, including double emulsions and gelled beads, and the level of control over the phases afforded by membrane emulsification was shown to be an advantage in the production of such microstructures.

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